The invention relates to a bus access unit for a pluggable electrical unit for the electrical connection of this pluggable electrical unit to an electrical base unit.
Complex electrical engineering systems, in particular in measuring and control technology and in telecommunications, are often of a modular construction. This involves a large number of electrical units being accommodated in an enclosure of an electrical base unit in a pluggable manner. The pluggable electrical units are mechanically of an identical construction but electrically different according to their intended function.
When a complex system of this type is expanded by adding further pluggable electrical units while operation is in progress or when one of the pluggable electrical units is exchanged, there is the risk of electrical components of the pluggable electrical unit and/or of the base unit being damaged or destroyed if an electrical unit which, though mechanically able to be inserted into the slot, is not electrically and functionally intended for this slot is inserted into the slot.
DE 196 10 556 discloses a bus access unit for connecting a subassembly of a programmable controller to a bus, which subassembly is spatially arranged in the base unit and is tested by means of a test contact and an evaluation circuit to ascertain that it is a plug-in module and, if it is a plug-in module, a variable resistor fitted into the feed line of the plug-in module is switched to low resistance after a predetermined waiting time.
Furthermore, EP 888 589 discloses a bus access unit for connecting a subassembly of a programmable controller to a bus, which subassembly is spatially arranged in the base unit and for which the bus lines of the control bus are connected via bidirectional inhibitable drivers to the terminals of the plug-in module which correspond to the bus lines. The bidirectional inhibitable drivers can be activated individually and in groups.
WO 97/07465 discloses a bus access unit with an adaptor plug, which for each slot is fitted between the base unit and the plug-in module, and has means for isolating the plug-in module from the base unit.
All the known bus access units have the common feature that the electrical interfaces of all the slots for receiving plug-in modules are uniform. Consequently, the electrical interfaces of electrically and functionally different plug-in modules have to be of an identical configuration.
Such uniform interfaces no longer allow individualizing of the electrical interface of the pluggable electrical units that is designed for the plug-in contact device, since plug-in contacts of the same name would be wired to different types of signals, which may also differ in the direction of signal transmission. If the free choice of slots is maintained for all the pluggable electrical units, the number of necessary plug-in contacts increases in proportion to the degree of individualization of the pluggable electrical units, and so does the complexity of the backplane wiring and of the bus access units. This additional complexity is felt to be disadvantageous. In addition, the overall size of the plug-in connector increases with the number of pins. Plug-in connectors of a larger overall size are at odds with the miniaturization of pluggable electrical units.
The invention is therefore based on the object of providing a bus access unit which is suitable for managing electrical interfaces of different types for electrically and functionally different plug-in modules while maintaining the free choice of slots in such a way that disruptions to units already in operation and damage to the unit to be inserted are avoided.
The invention is based on a bus access unit for a pluggable electrical unit for the electrical connection of this pluggable electrical unit to an electrical base unit, the base unit having a processing unit, and the bus access unit comprising a multipin plug-in contact device for the connection of the pluggable electrical unit to the base unit, a variable resistor, which is arranged in the feed line to the power supply of the pluggable electrical unit from the base unit, with a control input and with a high-resistance and a low-resistance state and means connected to the plug-in contact device for isolating control lines, data lines and address lines of the pluggable electrical unit from the base unit.
The essence of the invention is to arrange the variable resistor and the means for isolating control lines, data lines and address lines in the pluggable electrical unit and link them to one another in a causal chain.
It is specifically provided that the pluggable electrical unit has a first bus switch arrangement with a first control input for isolating a first group of control lines and for isolating signal lines for the identification of the pluggable electrical unit, the first control input of which is connected via a trigger circuit to the controlled terminal of the variable resistor.
The pluggable electrical unit additionally has a second bus switch arrangement with a second control input for isolating a second group of control lines, the second control input of which is connected to an isolated control line from the first group of control lines, a third bus switch arrangement with a third control input for isolating address lines, the third control input of which is connected to an isolated control line from the second group of control lines, and a fourth bus switch arrangement with a fourth control input for isolating data lines, the fourth control input of which is connected to an isolated control line from the second group of control lines.
This has the inevitable result that only when the plug-in contacts of the base unit leading to the supply voltage meet the corresponding mating plug-in contacts of the pluggable electrical unit can the pluggable electrical unit be supplied with power and the variable resistor can be brought into the low-resistance state.
Only in the low-resistance state of the variable resistor can the first bus switch arrangement be activated, after a predeterminable time period with an internal supply voltage present has elapsed, and the first group of control lines and the signal lines for the identification of the pluggable electrical unit can be switched through at low resistance by the plug-in contact device to further switching means of the pluggable electrical unit.
Only when the first bus switch arrangement has been successfully activated can the second bus switch arrangement be activated and the second group of control lines can be switched through at low resistance by the plug-in contact device to further switching means of the pluggable electrical unit.
Finally, only when the second bus switch arrangement has been successfully activated can the third and fourth bus switch arrangements be activated and consequently the address lines and the data lines can be switched through at low resistance by the plug-in contact device to further switching means of the pluggable electrical unit.
If the conditions mentioned are not satisfied, the respective bus switch arrangement remains at high resistance and the switching means of the pluggable electrical unit remain disconnected from those of the base unit. Because of this causal interrelationship, every pluggable electrical unit which, though it mechanically fits, is electrically incompatible remains isolated from the signal lines of the base unit. Consequently, disruptions to units already in operation and damage to the unit to be inserted are avoided.
According to a further feature of the invention, it is provided that the control input of the variable resistor is connected via contacts of the plug-in connection to the processing unit of the base unit. This achieves the effect that the power supply of a pluggable electrical unit identified as incompatible for the respective slots can be switched off.
A further advantage of the bus access unit according to the invention is that only the signal lines actually used in the pluggable electrical unit are isolated from those of the base unit. It is possible to dispense with the bus switches for the signal lines which, though routed from slot to slot in the base unit, are unused by the respective pluggable electrical unit. This reduces the complexity for the isolation of control lines, data lines and address lines of the pluggable electrical unit from the base unit.
In addition, by dispensing with unnecessary bus switches, the capacitive loading of the signal lines is reduced. While retaining the drive power, the signal-to-noise ratio for the signal lines is consequently improved. Further details and advantages of the invention are explained in more detail below on the basis of an exemplary embodiment.